The present invention relates to a thermoplastic resin. More particularly, the invention relates to a thermoplastic resin composition having excellent rigidity, moldability and surface gloss and improved impact resistance and environmental stress crack resistance.
There have been proposed various processes for improving impact strength or environmental stress crack resistance of thermoplastic resins by dispersing polymer particles having a rubbery elasticity therein. For example, rubber-modified thermoplastic resins such as ABS resin are prepared by the graft copolymerization of a polymer having a rubbery elasticity with a thermoplastic resin or by the polymer blending. In the preparation of the rubber-modified thermoplastic resins, a conjugated diene polymer; a copolymer of the conjugated diene monomer with a monomer such as styrene, acrylonitrile, methacrylate ester or acrylate ester; an amorphous olefin polymer; or a polymer having a quite low glass transition temperature such as an acrylate ester polymer is used as the rubbery polymer to improve the impact resistance and environmental stress crack resistance.
Various other processes have been proposed for improving the impact resistance or environmental stress crack resistance of said resin. For example, there have been proposed a process wherein the rubbery polymer content is increased, a process wherein a polymer having a low glass transition temperature is used, a process wherein a material having a large particle size is used and a process wherein a material having a large particle size is used in combination with a material having a small particle size. However, the degree of the improvement in impact resistance or environmental stress crack resistance has been limited, since physical properties such as rigidity, moldability and surface gloss are deteriorated by the treatment because of a low glass transition temperature of the rubbery polymer.
Further, there has been proposed a process for improving the environmental stress crack resistance by mechanically blending ABS resin with vinyl acetate polymer by an ordinary blending technique as disclosed in the specification of U.S. Pat. No. 3,883,614. However, the resin composition obtained by mechanically blending ABS resin with vinyl acetate polymer has yet insufficient environmental stress crack resistance even after the treatment and its impact strength is not yet improved.
There has also been known a rubber-modified thermoplastic resin obtained by copolymerizing ethylene with vinyl acetate in the presence of a diene rubber polymer and then copolymerizing the resulting graft copolymer latex with styrene and acrylonitrile. However, this resin has an insufficient polymerization stability and the improvement in environmental stress crack resistance thereof is yet unsatisfactory.